Apply t.timestamps changes in Action Text and Action Mailbox

Follow up #34956.

actionmailbox/db/migrate/20180917164000_create_action_mailbox_tables.rb

@@ -5,11 +5,7 @@ def change
       t.string  :message_id, null: false
       t.string  :message_checksum, null: false
 
-      if supports_datetime_with_precision?
-        t.timestamps precision: 6
-      else
-        t.timestamps
-      end
+      t.timestamps
 
       t.index [ :message_id, :message_checksum ], name: "index_action_mailbox_inbound_emails_uniqueness", unique: true
     end

actiontext/db/migrate/201805281641_create_action_text_tables.rb

@@ -5,8 +5,7 @@ def change
       t.text       :body, limit: 16777215
       t.references :record, null: false, polymorphic: true, index: false
 
-      t.datetime :created_at, null: false
-      t.datetime :updated_at, null: false
+      t.timestamps
 
       t.index [ :record_type, :record_id, :name ], name: "index_action_text_rich_texts_uniqueness", unique: true
     end